Altera SerialLite II IP Core User Manual
Page 40
Chapter 3: Parameter Settings
3–17
Link Layer Configuration
January 2014
Altera Corporation
SerialLite II MegaCore Function
User Guide
When flow control is enabled, the SerialLite II MegaCore logic monitors the triggering
receive FIFO buffer and, when a threshold is reached, issues a pause instruction. It
takes some time for the pause instruction to be issued, traverse the connection, and for
transmission to be stopped. It takes more time for all the data that has already been
transmitted to be stored in the receive FIFO buffer. Therefore, there must be a certain
amount of space left in the receive FIFO buffer above the threshold to hold the data
that arrives during this delay. This headroom has contributions from the core latency
and the wire latency. Refer to section
“Flow Control Operation Example” on
” for more details.
If the far receive FIFO buffer is still in breach of the threshold when the flow control
refresh period timer expires, the far receiver automatically renews the pause to extend
the flow control period. This renewal occurs until the fill level of far receive FIFO is no
longer greater than the threshold. When the renewed flow control packet reaches the
near transmitter before the current pause expires, the pause time is refreshed.
■
This refresh time must be set so that the renewed flow control packets are received
by the near transmitter before the current pause time completes. Set the value of
Refresh period
to be smaller than Pause quantum time in the Priority Packet
Settings
or Data Packet Settings section on the Link Layer page.
■
If the refresh period is small, more flow control packets are sent on the link,
possibly degrading the performance of an alternate active port. This is a trade off
for the link bandwidth performance.
To overcome head-of-line blocking, every port has its own flow control that suspends
the flow of data to either the priority port or the regular data port, depending on the
FIFO buffer status. For example, if the near transmitter receives a flow control pause
request for the priority port, the data on the regular port is transmitted (as long as the
regular port is not also being requested to pause).